Commitments and contingencies - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Dec. 31, 2024
Loss Contingencies [Line Items]
Commitments to extend credit to commercial customers	$ 12,800,000		$ 12,700,000
Maximum credit risk for recourse associated with loans sold under Federal National Mortgage Association Delegated Underwriting and Servicing program	4,400,000		4,200,000
Loss contingency accrual	75,000		$ 157,000
Loss contingency accrual, period decrease	8,000
Estimate assessed losses		$ 34,000
Minimum
Loss Contingencies [Line Items]
Range of reasonably possible losses	0
Maximum
Loss Contingencies [Line Items]
Range of reasonably possible losses	$ 25,000